This invention relates to development systems for electrostatographic processors and, more particularly, to hybrid crossmixers for development systems which employ multi-component developer mixtures.
In a conventional electrostatographic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography", a uniformly charged imaging surface is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided coloring material, called "toner". As is known, that process has enjoyed outstanding commercial sources, especially in plain paper xerographic copiers and duplicators. However, it is not limited to xerography or to use in stand alone copiers and duplicators. For example, there are electrostatographic processors which have appropriately controlled stylii for forming the latent electrostatic image on the imaging surface. Furthermore, it has been found that electrostatographic printing may be advantageously utilized in facsimile systems and computer printers, to name just some non-copier applications.
Multi-component developer mixtures are a favored vehicle for providing the necessary toner to render electrostatic latent images visible. A developer of this type contains a mixture of toner particles and larger, so-called "carrier" particles. In practice, the materials for the toner and carrier (or, sometimes, carrier coating) components of the mixture are selected so that they are removed from one another in the triboelectric series, whereby electrical charges of opposite polarities tend to be imparted to the toner and carrier particles when the developer components are blended together. Moreover, in selecting those materials, consideration is given to their triboelectric ranking to the end that the nominal polarity for the charge of the toner particles opposes the polarity of the latent images which are to be developed. Consequently, in operation, there are competing electrostatic forces acting on the toner particles. That is, one set of forces tends to attract the toner particles to the carrier particles, while another set of forces tends to electrostatically strip toner from that portion of the developer which is brought into the immediate proximity of or actual contact with the image bearing imaging surface.
Cascade and magnetic brush development systems have both been used with substantial success in continuous electrostatographic processors. Characteristically, those systems circulate a multi-component developer from a sump, through a development zone, and then back to the sump. The aim, of course, is to develop the latent images carried by the imaging surface on the fly -- viz., as the imaging surface moves through the development zone. Hence, there typically is a toner dispenser for adding additional toner to the developer from time-to-time so that the toner concentration of the developer remains at a suitably high level. Furthermore, there normally is a crossmixer for mixing and blending the components of the developer.
As used herein, "mixing" refers to redistributing the developer components in order to maintain a more or less uniform distribution of the available toner. In contrast, "blending" refers to agitating the developer in order to promote the triboelectric charging of the toner and carrier particles.
Surprisingly, most of the available crossmixers still show a weakness when measured by their ability to carry out the dual functions of mixing and blending a multi-component developers. As a general rule, passive crossmixers are effective mixing devices, but only marginally acceptable as blending devices. Active crossmixers, on the other hand, are normally effective blending devices, but only marginally acceptable as mixing devices. The crossmixer described and claimed in my copending and commonly assigned United States Patent application which was filed Feb. 27, 1975 now U.S. Pat. No. 3,943,887 is a distinct improvement, but there still is room for further improvement.